A prickly predicament for Britain's hedehogs

The People’s Trust for Endangered Species and the British Hedgehog Preservation Society warn that the impact of environmental change on this generalist species may be indicative of more profound impacts elsewhere in our ecosystems....

Conservation efforts are led by an understanding of how wild populations change, but as mammals are typically nocturnal and unshowy, surveys of them over sizeable areas are difficult, particularly in urban areas of green mosaics characterised by multiple uses and owners.

Such spaces, however, may be important for wildlife conservation. Indeed, more than at any time in the past, they may be especially significant for hedgehogs (Erinaceus europaeus), which have lost out owing to changes in the wider agricultural landscape.

A ten-year study by the People’s Trust for Endangered Species (PTES) was conducted by over 2,000 volunteers recording sightings and signs of mammals in the built environment. Its results painted a less than rosy picture. Concern for hedgehogs was such that PTES and the British Hedgehog Preservation Society campaigned extensively to help the species.

Between 2003 and 2012 records of hedgehogs were collected from around 2,500 sites in the green infrastructure of towns and communities – gardens, parks, cemeteries, allotments, commons, etc. Cumulatively, these records represented an unusually helpful conservation tool - a dataset covering a period long enough to identify trends beyond mere annual fluctuations. Taking 2004 as the base-line, the proportion of ‘positive’ sites (ie those recording hedgehogs) decreased by a quarter, equivalent to an average fall of 3.7 per cent each year.

Given that statistic alone, it’s possible the trend reflects a loss of suitable habitat and that a hedgehog population of static size is being squeezed into a diminishing number of sites. If such was the case, more hedgehogs would be expected at sites where they remained – in fact, the number of hedgehogs recorded showed a slightly greater annual decline of 5.9 per cent. Taken together, the decline in both the number of positive sites and quantity of hedgehogs is significant; if the species was a bird, the RSPB would accord it Red List status - the highest conservation priority.

Further afield, the evidence from other studies is just as damning. Summarised in The State of Britain’s Hedgehogs 2011, the results of five surveys analysed in a report by the British Trust for Ornithology (BTO) give the most complete picture yet of a wider decline. Two surveys which record hedgehogs in the rural landscape, BTO’s Breeding Bird Survey (which, despite its title, includes mammal records) and PTES’ Mammals on Roads survey, indicate annual declines of around 8 and 2.5 per cent, going back to 1996. Between 2001 and 2011, counts of hedgehogs on roads outside towns fell by over a quarter.

Not only should alarm bells be ringing for Britain’s hedgehogs, but their loss raises wider questions about the environment. As David Wembridge, of PTES, puts it; “Hedgehogs are generalists, they feed on soil invertebrates and they're not very fussy in habitat requirements, so if there is a big decline in hedgehogs, it raises concerns about the quality of the environment.”

While all the surveys point to an annual loss of several per cent – comparable to the rate at which the world’s tiger population is dwindling – the size of the loss historically is harder to gauge. In most cases, estimating the ‘census’ number of a wild population comes down to a best guess based on the extent of available habitat and typical densities of animals in particular habitats.

One estimate of the British hedgehog population in the 1950s was 36.5 million, but it’s difficult to be confident of its accuracy. With a little more confidence, the 1995 figure was put at 1.5 million, based on which there are now fewer than a million.

Reasons for the decline almost certainly involve habitat loss and its fragmentation. Also to blame may be a poorer quality habitat, with meagre hedgerows and less invertebrate prey. In urban areas, housing developments with smaller gardens and impenetrable fencing results in such green spaces becoming inaccessible to hedgehogs. In suburban areas male hedgehogs may forage nightly for up to two or three kilometres, so it’s necessary to consider urban habitats on a neighbourhood-scale if hedgehogs’ chances are to be improved.

A changing climate may also have an effect on hedgehog numbers. As Pat Morris, a doyen of hedgehog research, explains: “Age, sex and weather all appear to influence the timing of hedgehog hibernation. Mild weather can.….delay hedgehogs entering into hibernation or elicit premature awakening, impacting on the creature’s fat reserves and breeding times, and consequently affecting the long-term survival of the species.”

A study by Morris in the 1970s found that hedgehogs living in different parts of the British Isles had quite different behavioural schedules. For example, hedgehogs emerged from hibernation up to three weeks earlier in southwest England than in Scotland. Similarly, animals living in East Anglia spent longer hibernating than those in London or the south-west. Latitude and local climate seem to play a part in hedgehog ecology.

Intuitive though these differences may seem, when the methodology was repeated in 2012, the regional patterns seen in the 1970s were not apparent. Surprisingly, for example, Scottish hedgehogs were out and about significantly earlier than their southern counterparts. It’s unclear whether this was caused by an unusually mild spring or another factor, as yet unidentified.

Once again, a long dataset is neccessary, as only by repeatedly collecting records over successive years is it possible to account for the natural stochasticity that affects all populations. This is especially important when attempting to disentangle the effects of weather from those of the broader, changing climate.

Timing is important, and the phenotypology of British plants and animals tells a story of shifting relationships. An accurate map of regional variations in hedgehog activity will help in making informed decisions about their habitat management, rescue, and rehabilitation. It will also increase the chances of helping this extraordinary spiny mammal adapt to an uncertain future.